Fix crash when parsing karaoke data from a line without an extra field

src/auto4_lua_assfile.cpp

@@ -331,6 +331,10 @@ namespace Automation4 {
 			else if (type != LUA_TNIL) {
 				error(L, "dialogue extradata must be a table");
 			}
+			else {
+				// rebalance the stack after lua_getfield pushed a nil onto it
+				lua_pop(L, 1);
+			}
 		}
 		else {
 			error(L, "Found line with unknown class: %s", lclass.c_str());

tests/meson.build

@@ -34,6 +34,22 @@ tests_src = [
     'tests/line_iterator.cpp',
     'tests/line_wrap.cpp',
     'tests/lua_lfs.cpp',
+    'tests/lua_parse_karaoke_data.cpp',
+
+    # parse_karaoke_data lives in src rather than libaegisub, so the pieces it needs are compiled
+    # into the test binary alongside it
+    '../src/ass_attachment.cpp',
+    '../src/ass_dialogue.cpp',
+    '../src/ass_entry.cpp',
+    '../src/ass_file.cpp',
+    '../src/ass_karaoke.cpp',
+    '../src/ass_override.cpp',
+    '../src/ass_style.cpp',
+    '../src/ass_style_storage.cpp',
+    '../src/auto4_lua_assfile.cpp',
+    '../src/compat.cpp',
+    '../src/utils.cpp',
+
     'tests/mru.cpp',
     'tests/option.cpp',
     'tests/path.cpp',
@@ -64,7 +80,7 @@ endif
 runner = executable(
     'gtest-main',
     tests_src,
-    include_directories: [tests_inc, libaegisub_inc],
+    include_directories: [tests_inc, libaegisub_inc, include_directories('../src')],
     dependencies: [tests_deps, deps],
     cpp_args: tests_args + conf_defines,
     link_with: libaegisub,

tests/tests/lua_parse_karaoke_data.cpp

@@ -0,0 +1,123 @@
+// Copyright (c) 2026, Aegisub Project http://www.aegisub.org/
+//
+// Permission to use, copy, modify, and distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+// ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+// ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+// OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+//
+// Aegisub Project http://www.aegisub.org/
+
+#include <main.h>
+
+#include <ass_file.h>
+#include <auto4_lua.h>
+
+#include <libaegisub/mru.h>
+#include <libaegisub/option.h>
+#include <libaegisub/path.h>
+
+#include <lua.hpp>
+#include <string>
+
+// The application-wide state these name is set up by the entry point, which a test binary cannot
+// link. Only paths this test never takes reach them — loading a default subtitle file, resolving a
+// style catalog, recording a recently used file — so definitions are all the linker wants.
+namespace config {
+	agi::Options *opt = nullptr;
+	agi::MRUManager *mru = nullptr;
+	agi::Path *path = nullptr;
+}
+
+namespace {
+// `parse_karaoke_data` is registered onto the aegisub table by LuaAssFile's constructor, so a
+// LuaAssFile has to exist for the whole call and the table has to be there before it is built.
+class lua_parse_karaoke_data : public ::testing::Test {
+protected:
+	lua_State *L = nullptr;
+	AssFile file;
+
+	void SetUp() override {
+		L = lua_open();
+		ASSERT_NE(nullptr, L);
+		luaL_openlibs(L);
+
+		lua_newtable(L);
+		lua_setglobal(L, "aegisub");
+	}
+
+	void TearDown() override {
+		if (L) lua_close(L);
+	}
+
+	/// Runs a chunk that calls parse_karaoke_data and concatenates the `text_stripped` of every
+	/// syllable it returned past the index-zero filler. Fails the test if the call raises or hands
+	/// back something other than a table.
+	std::string syllable_text(const char *chunk) {
+		new Automation4::LuaAssFile(L, &file, true, false);
+
+		if (luaL_loadstring(L, chunk)) {
+			ADD_FAILURE() << "failed to compile chunk: " << lua_tostring(L, -1);
+			lua_pop(L, 1);
+			return {};
+		}
+
+		if (lua_pcall(L, 0, 1, 0)) {
+			ADD_FAILURE() << "parse_karaoke_data failed: " << lua_tostring(L, -1);
+			lua_pop(L, 1);
+			return {};
+		}
+
+		if (lua_type(L, -1) != LUA_TTABLE) {
+			ADD_FAILURE() << "expected a table, got " << lua_typename(L, lua_type(L, -1));
+			lua_pop(L, 1);
+			return {};
+		}
+
+		std::string ret;
+		for (int i = 1; i <= (int)lua_objlen(L, -1); ++i) {
+			lua_rawgeti(L, -1, i);
+			lua_getfield(L, -1, "text_stripped");
+			if (lua_type(L, -1) == LUA_TSTRING)
+				ret += lua_tostring(L, -1);
+			lua_pop(L, 2);
+		}
+		lua_pop(L, 1);
+		return ret;
+	}
+};
+
+// Everything LuaToAssEntry's dialogue branch reads, with the optional extradata left to the caller.
+const char line_fields[] =
+	"class = 'dialogue', comment = false, layer = 0, "
+	"start_time = 0, end_time = 5000, style = 'Default', actor = '', "
+	"margin_l = 0, margin_r = 0, margin_t = 0, effect = '', ";
+}
+
+TEST_F(lua_parse_karaoke_data, splits_syllables) {
+	std::string chunk = std::string("return aegisub.parse_karaoke_data{") + line_fields +
+		"text = '{\\\\k20}ka{\\\\k30}ra', extra = {}}";
+	EXPECT_EQ("kara", syllable_text(chunk.c_str()));
+}
+
+// A previous bug in LuaToAssEntry's dialogue branch left a nil on the stack when no `extra` field
+// was present on a line table passed to `aegisub.parse_karaoke_data(line)`, e.g. via the public
+// `karaskel.preproc_line_text(meta, styles, line)` API, which in turn caused LuaParseKaraokeData
+// to try writing syllables into a nil rather than a table, resulting in a crash.
+TEST_F(lua_parse_karaoke_data, accepts_a_line_without_extradata) {
+	std::string chunk = std::string("return aegisub.parse_karaoke_data{") + line_fields +
+		"text = 'Hello world'}";
+	EXPECT_EQ("Hello world", syllable_text(chunk.c_str()));
+}
+
+TEST_F(lua_parse_karaoke_data, splits_syllables_without_extradata) {
+	std::string chunk = std::string("return aegisub.parse_karaoke_data{") + line_fields +
+		"text = '{\\\\k20}ka{\\\\k30}ra'}";
+	EXPECT_EQ("kara", syllable_text(chunk.c_str()));
+}